Re: [PATCH v4 2/2] kho: Introduce preserve/restore APIs for high-order pages

[Pranjal Shrivastava <praan@google.com>]

On Wed, Aug 12, 2026 at 01:10:23PM +0200, Pratyush Yadav wrote:
> On Mon, Aug 03 2026, Pranjal Shrivastava wrote:
> 
> > The current KHO page preservation APIs (e.g. kho_preserve_pages) assume
> > that multi-page blocks are split into independent 4KB pages during
> > restoration. This is incompatible with high-order non-compound pages,
> > such as DMA buffers, which must be restored with tail pages having a
> > zero reference count.
> >
> > Introduce explicit preserve and restore APIs for high-order pages,
> > which preserve and restore a high-order page block as a single unit,
> > applying a refcount of 1 to the head page while leaving tail pages at 0.
> > Rename the existing internal helper to __kho_restore_page() and
> > consolidate the common restoration code into it.
> >
> > Signed-off-by: Pranjal Shrivastava <praan@google.com>
> 
> The code here looks very convoluted TBH. I think it will be simpler to
> make kho_restore_page() only return non-compound pages. That is, it
> returns 0 or higher order non-compound page.
> 
> Then kho_restore_pages() can call kho_restore_page() and then do
> split_page() on the page it got to turn it into 0-order pages.
> kho_restore_folio() can call kho_restore_page() and then do
> prep_compound_page() on the page it got.
> 
> And then you expose kho_restore_page() to be used by DMA APIs to get
> non-compound high order pages directly.
> 
> How does that sound?
> 

I like the suggestion to use a single base primitive and mold it via
split_page() and prep_compound_page().

However, To implement this, we'll need __kho_restore_page(phys, &order)
as the internal base primitive because callers (like kho_restore_folio)
need to retrieve the order from the KHO metadata to pass into 
prep_compound_page(), and a raw non-compound struct page doesn't store 
its order.

By having __kho_restore_page() initialize the head_ref = 1 & all_tails
=0 refcount pattern by default, all three public APIs can just wrap it
and call the appropriate MM subsystem helpers. I'll spin v5 with this

Thanks,
Praan